* Charles McCathieNevile <charles@w3.org> [2004-03-12 11:34-0500]
>
> It isn't clear from that explanation what the difference is, since unless you
> talk about the domain and range of foaf:topic it's hard to understand how
> there is any.
dc:subject relates a document to a code that stands for some thing the
doc is about.
foaf:topic relates a document to some thing that the doc is about.
...the difference is w.r.t. layers of indirection: where dc:subject uses
an external taxonomy, set out in advance, foaf:topic relies on any chunk
of RDF that is handy for describing the thing. It's an interesting
stylistic and representational difference that seems really quite
unfortunately hard to explain clearly...
>
> The fact that foaf:topic has a defined domain of foaf:Document and a range of
> rdf:Resource means that it's a bit more restricted than dc:subject which can
> happily live with a literal value, but there doesn't seem to be much else
> that can be used to pick between them (on my reading of the two sets of
> specifications).
They differ importantly, just as you differ from your homepage, and XML
differs from library subject codes for XML. With dc:subject there is a
whole other world squeezed into the representation: the world of
subject/topic codes. With foaf:topic, the RDF itself does that work,
removing subject/topic codes from the list of things the RDF needs to
describe. See above re this being hard to describe :(
>
> If I was trying to define the way I see the web I would write somewhere that
> foaf:topic seems to me like a subProperty of dc:subject.
that would imply that any pair of things related by foaf:topic are also
related by dc:subject. This isn't so, since the values taken by
dc:subject are various forms of subject code, whereas the values taken
by foaf:topic are the things that those subject codes denote. So
declaring a subPropertyOf relation would confuse those two levels.
> Being a believer in
> living language, and in language as an attempt to provide identifiers for
> concepts in such a way thhat we are convinced we mean the same thing, this
> doesn't strike me as a bad thing to do. But since we are inventing RDF
> vocabulary creation, it is something I would treat with care - especially
> while we don't yet have good methods for dealing with conflicting statements,
> including conflicting statements about how vocaublaries are defined.
We defintely need to to allow for vocabulary evolution, it might be
interesting to compare the two approaches...
Dan